1. Field of the Invention
The present invention generally relates to a method and apparatus for efficiently using a control channel to achieve a maximum interference diversity in a wireless communication system. More particularly, the present invention relates to a method and apparatus for efficiently mapping a Physical Downlink Control CHannel (PDCCH) to time-frequency resources in an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system.
2. Description of the Related Art
Recently, Orthogonal Frequency Division Multiplexing (OFDM) has been actively studied as a promising means for high-speed data transmission on wired/wireless channels in mobile communication systems. OFDM is a special case of Multi-Carrier Modulation (MCM) in which an input serial symbol sequence is converted to parallel symbol sequences and modulated to orthogonal subcarriers, i.e., subcarrier channels. OFDMA is a multi-user version of OFDM, in which multiple users are distinguished by subcarriers, i.e., different subcarriers are allocated to different users.
FIG. 7 illustrates a method for transmitting data to a plurality of users in a conventional OFDMA system.
Referring to FIG. 7, the horizontal axis represents time and the vertical axis represents frequency. Reference numeral 701 denotes a time-domain resource allocation unit, usually including a plurality of OFDM symbols. This resource allocation time unit is referred to as a subframe. Reference numerals 702, 703 and 704 denote first, (N−1)th, and Nth bands when a total system band is divided into N bands. Each band generally includes a plurality of subcarriers. As illustrated in FIG. 7, typically, a plurality of resource blocks are formed with time and frequency resources, and resources are allocated to a plurality of users in resource blocks in an OFDM system. A single resource block is equivalent to the resources defined by one subframe and one band in FIG. 7. For example, if one band includes 12 subcarriers and one subframe is comprised of 14 OFDM symbols, one resource block has 168 (12×14) time and frequency resources. A minimum time and frequency resource unit, i.e., one subcarrier in one OFDM symbol is referred to as a Resource Element (RE).
As described above, a Node B allocates resources to at least one User Equipment (UE) in units of resource blocks in every subframe and notifies the UEs of the resource allocation results by resource allocation information in every subframe in the OFDM system. A channel carrying the resource allocation information is a shared control channel or a PDCCH. Hereinafter, a PDCCH is referred to as a control channel. In general, the control channel may include other information in addition to the resource allocation information, which will not be described in detail herein. Information including the resource allocation information that the control channel delivers is called control information. A single control channel carries resource allocation information about a single UE. Hence, when resources are allocated simultaneously to a plurality of UEs in a subframe, a plurality of control channels is transmitted to the UEs.
Generally, control channels that the Node B generates for a plurality of users are mapped to REs in Time Division Multiplexing/Frequency Division Multiplexing (TDM/FDM) after channel coding and interleaving, prior to transmission to the users. The control channels are mapped in the same manner for each Node B and for each subframe. Resource mapping refers to mapping the control channels to physical REs. For example, if five control channels are transmitted in a subframe, the five control channels are mapped to REs in the same manner in every Node B. Therefore, when interference occurs between control channels from different Node Bs, the absence of randomization effects may cause performance degradation. In other words, any control channel is susceptible to some statistically strong interference, resulting in performance degradation.